Re: [whatwg/dom] Introduce `moveBefore()` state-preserving atomic move API (PR #1307)

@domfarolino commented on this pull request.



> + <li><p>Let <var>oldParent</var> be <var>node</var>'s <a for=tree>parent</a>.
+
+ <li><p><a>Assert</a>: <var>oldParent</var> is non-null.
+
+ <li><p>Let <var>index</var> be <var>node</var>'s <a for=tree>index</a>.
+
+ <li>
+  <p>For each <a>live range</a> whose <a for=range>start node</a> is <var>parent</var> and
+  <a for=range>start offset</a> is greater than <var>index</var>, decrease its
+  <a for=range>start offset</a> by 1.</p>
+
+  <p class="note">Note that unlike the traditional <a for=/ lt="remove">removal</a> case, we do not
+  need to update <a>live range</a> state when their <a for=range>start node</a> or
+  <a for=range>end node</a> is an <a>inclusive descendant</a> of the <var>node</var>. This is
+  because said <a>nodes</a> do not get removed from their <a>tree</a>, so ranges associated with
+  them stay intact.</p>

I mean that it wouldn't be able to "preserve" selection at the previous start and end containers. Ordinarily, we can't preserve selection anchored at those containers, since those containers get removed from the DOM. But with `moveBefore()`, those containers never leave the DOM, so we have an opportunity to do something better by keeping ranges anchored at those containers, which is a higher-fidelity preservation.

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